1. Field of the Invention
The present invention relates generally to a brake pipe arrangement for an automotive vehicle and more specifically to a brake pipe arrangement which can reduce abnormal oscillation of brake units.
2. Description of the Prior Art
In usual, vehicle brake can be obtained by supplying hydraulic pressure generated from a master cylinder when a brake pedal is depressed to brake cylinders of brake units provided for front and rear wheels. The brake pipe arrangement for supplying hydraulic pressure to the brake units can be classified into two, front-rear split type and diagonal split type. In the above front-rear split type, a front wheel pipe connected to a front hydraulic pressure port of a master cylinder is branched into two, right and left, front wheel brake cylinders, while a rear wheel pipe connected to a rear hydraulic pressure port of the master cylinder is branched into two, right and left, rear wheel brake cylinders.
In the prior-art brake pipe arrangement for an automotive vehicle, the length of the split pipes extending from the split point to each brake cylinder is roughly equal to each other; that is, a difference in brake pipe length between the right and left wheel brake cylinders is relatively small from the vehicle structural standpoint. Therefore, when a wall thickness of a brake rotor is worn away and therefore brake torque of one brake unit varies, hydraulic pressure fluctuates or vibrates, so that the brake system tends to vibrate at a low frequency (e.g. 10 to 15 Hz). The above hydraulic pressure vibration is transmitted from the right brake unit cylinder to the left brake unit cylinder or vice versa via the two split pipes.
FIG. 1(A) shows the prior-art relationship between the frequency and the phase difference in hydraulic pressure between the two right and left brake cylinders, in which the dot-dashed line indicates the phase difference (degree) obtained when the hydraulic pressure changes in the right brake cylinder and then the changed pressure is transmitted to the left brake cylinder, and the dot-dot-dashed line indicates the phase difference (degree) obtained when the hydraulic pressure changes in the left brake cylinder and then the changed pressure is transmitted to the right brake cylinder. The small difference between the two lines results from a difference in length of pipes extending from the master cylinder (or the branch point) to the two right and left brake cylinders.
Further, FIG. 1(B) shows the prior-art relationship between the frequency and the hydraulic pressure change in the two right and left brake cylinders, in which the dot-dashed line indicates the pressure change (dB) obtained when the hydraulic pressure changes in the right brake cylinder and then the changed pressure is transmitted to the left brake cylinder, and the dot-dot-dashed line indicates the pressure change (dB) obtained when the hydraulic pressure changes in the left brake cylinder and then the changed pressure is transmitted to the right brake cylinder. Further, in FIG. 3(B), the dashed line indicates the relationship between frequency and torque change in one brake cylinder obtained when torque changes in the other brake cylinder.
With reference to FIGS. 1(A) and (B), since the phase difference between the two cylinders is about 90 degrees at 10 Hz (brake cylinder hydraulic pressure vibration frequency), the pressure change is relatively low. However, when the vibration frequency increases from 10 Hz to 20 Hz, since the phase difference increases from 90 to 180 degrees, the pressure change also increases up to the peak resonant point R.sub.0 as shown in FIG. 1(B).
On the other hand, since this hydraulic pressure vibration frequency range from 10 to 18 Hz corresponds to the frequency range during which brake shimmy (an abnormal oscillation in the front wheels of a motor vehicle reinforced by resonance at critical speeds) is generated, there exists a problem in that the brake shimmy phenomenon is accelerated and therefore there exists a need of eliminating hydraulic pressure fluctuations or torque vibration; that is, it is necessary to strictly match the thickness of one side brake rotor to that of the other side brake rotor. In other words, in the prior-art brake units, it has been necessary to often replace the brake units with a new set of brake units, thus resulting in poor workability and higher cost.